I can confirm that this is an issue if you do not have a flat or deck placed next to the wall where you've placed the columns.
If you can place a flat or deck, the columns only show up outside, although their shadows show up inside (but that's for another thread!):
It appears to be happening because the open foundation area is no longer considered a proper room. You will notice that the same thing happens if you simply draw a wall and place columns on it.
I don't know if I would consider this a bug. It really relates more to the game's design of treating what it defines as a "room" in one manner and having different rules for everything else, and the fact that once you carve out that foundation space (or make an open story area), you have lost the "room" designation. Until/unless they change that design, you're going to have stuff like this (and lighting, and roofs intruding into interiors, etc.).
Thank you so much for replying!
I will change the design.
I also appreciate for attaching photos (they helped me greatly to understand your point).
